The present invention relates to a method of writing bit sequences as a self-timing signal wave on a magnetic recording medium and/or for reading the written signal wave.
The mechanical and electrical properties of a given magnetic memory require a specific minimum spacing between flux changes if information is to be recorded and reproduced with sufficient reliability. From this minimum spacing between flux changes follows the maximum density of flux changes (density of flux changes = number of changes of magnetization over a given length of track). It is a reference magnitude for the storage capacity. Another reference magnitude is the efficiency of the recording mode used. The efficiency is the ratio of the bit density (= number of bits stored along a given length of track) to the density of flux changes. It is equal to the reciprocal of the number of minimum spaces between flux changes required to store one bit.
The two-frequency recording mode according to DIN 66 010 has an efficiency of 50%, and the three-frequency recording mode according to U.S. Pat. No. 3,414,894 has an efficiency of 100%.
In the three-frequency recording mode, use is made of three different flux-change spacings two of which have different meanings. Thus, the information cannot be recovered from the spacing alone, it is also necessary to determine the instant of the flux change in relation to the clock period or, in other words, the meaning of a bit is dependent on the preceding bit in the bit sequence. This means that a bit sequence to be written must always be preceded by a particular bit or a particular bit pattern in order that the read circuit can decode the bit sequence at all, and that a dropout of a flux change results in the following information being decoded wrongly.